Field of the Invention
The present invention relates to a honeycomb filter. More particularly, it relates to a honeycomb filter to be suitably used in purification of particulate matter included in an exhaust gas from an engine, especially a car engine, and toxic gas components such as nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC).
Description of the Related Art
In recent years, reductions of fuel consumptions of cars have been required from the viewpoints of influences on the global environment and resource saving. Consequently, there is the tendency that internal combustion engines such a direct injection type gasoline engine and a diesel engine each having an excellent thermal efficiency are used as power sources for the cars.
On the other hand, in these internal combustion engines, generation of ashes generated during combustion of fuel raises problems. From the viewpoint of the air atmosphere, there are required countermeasures for removal of toxic components included in the exhaust gas and simultaneously, prevention of emission of particulate matter (hereinafter referred to as “the PM” sometimes) such as soot or ash to the atmosphere.
In particular, there is the tendency that regulations on the removal of the PM emitted from the diesel engine are strengthened worldwide, and use of a honeycomb filter as a trapping filter to remove the PM (hereinafter referred to as “the DPF” sometimes) attracts attention. Further, there have been suggested various exhaust gas purification systems in which such a honeycomb filter is used. The above DPF usually has a structure where a plurality of cells which become through channels for a fluid are usually defined by porous partition walls and cells are alternately plugged, whereby the porous partition walls constituting the cells perform a role of the filter.
In the DPF, the exhaust gas containing the particulate matter and the like flow in the DPF from a first end face (an inflow end face), the particulate matter is filtered with the partition walls, and then, the purified gas flows out from a second end face (an outflow end face). In such a DPF, there has been the problem that, with the inflow of the exhaust gas, the particulate matter contained in the exhaust gas is deposited on the partition walls to close the cells on an exhaust gas inflow side. This is a phenomenon which easily occurs in a case where a large amount of the particulate matter is contained in the exhaust gas, or in cold districts. When the cells are closed in this manner, the problem occurs that a pressure loss in the DPF rapidly enlarges. Therefore, to inhibit such cell closing, the filter is contrived to increase a filtration area or an open frontal area in the exhaust gas inflow side cells.
As the above structure of the DPF, specifically, a structure has been suggested in which a sectional area of each inflow side cell is different from a sectional area of each outflow side cell (e.g., see Patent Document 1). Here, the sectional area of each cell indicates the area of the cell in a cross section when the cell is cut along a plane perpendicular to a central axis direction of the cell. The inflow side cells are cells which are opened in the inflow end face and whose open ends in the outflow end face are plugged by plugging portions, and the cells are also referred to as outlet plugged cells. On the other hand, the outflow side cells are cells whose open ends in the inflow end face are plugged by the plugging portions and which are opened in the outflow end face, and the cells are also referred to as inlet plugged cells. Additionally, hereinafter, the structure in which the sectional area of each inflow side cell is different from the sectional area of each outflow side cell will be referred to as “the HAC structure” sometimes. “The HAC” is abbreviation for High Ash Capacity.
In addition, there has been suggested a honeycomb filter of the HAC structure which has inflow side cells whose sectional areas are large and outflow side cells whose sectional areas are small and in which a sectional shape of each inflow side cell is different from a sectional shape of each outflow side cell (e.g., see Patent Document 2). Here, the sectional shape of the cell is a shape which appears in a cross section of the cell when the cell is cut along a plane perpendicular to a central axis direction of the cell.
In addition, as another structure of the DPF, a structure has been suggested in which a periphery of a trapping cell group constituted of a plurality of inflow side cells (outlet plugged cells) is surrounded with a plurality of outflow side cells (inlet plugged cells) (e.g., Patent Document 3).
To use the honeycomb filter continuously for a long period of time, it is necessary to periodically subject the honeycomb filter to a regeneration treatment. That is, for the purpose of reducing the pressure loss enlarged due to the soot deposited in the honeycomb filter with an elapse of time to return a filter performance to an initial state, it is necessary to burn and remove the soot deposited in the honeycomb filter by a high-temperature gas. To smoothly perform such a regeneration treatment, a catalyst to burn and remove the soot is loaded onto the honeycomb filter in a certain case. As this catalyst, a noble metal such as platinum or palladium is used. Hereinafter, the burning and removal of the soot deposited in the honeycomb filter will simply be referred to as “regeneration” of the honeycomb filter sometimes.
[Patent Document 1] WO2009/069378
[Patent Document 2] JP-A-2004-000896
[Patent Document 3] JP-A-2010-053697